One of the most important functions of a wellbore fluid is to contribute to the stability of the wellbore, and control the flow of gas, oil or water from the pores of the formation in order to prevent, for example, the flow or blow out of formation fluids or the collapse of pressured earth formations. The column of fluid in the hole exerts a hydrostatic pressure proportional to the depth of the hole and the density of the fluid. High-pressure formations may require a fluid with a specific gravity of up to 3.0.
A variety of materials are presently used to increase the density of wellbore fluids. These include dissolved salts such as sodium chloride, calcium chloride and calcium bromide. Alternatively powdered minerals such as barite, calcite and hematite are added to a fluid to form a suspension of increased density. It is also known to utilize finely divided metal such as iron as a weight material. In this connection, the literature discloses a drilling fluid where the weight material includes iron/steel ball-shaped particles having a diameter less than 250 μm and preferentially between 15 and 75 μm. It has also been proposed to use finely powdered calcium or iron carbonate however the difficulty is that the plastic viscosity of such fluids rapidly increases as the particle size decreases.
It is a requirement of wellbore fluids that the particles form a stable suspension, and do not readily settle out. A second requirement is that the suspension should exhibit a low viscosity in order to facilitate pumping and to minimize the generation of high pressures. Another requirement is that the wellbore fluid slurry should exhibit low filtration rates (fluid loss).
Conventional weighting agents such as powdered barite exhibit an average particle diameter (d50) in the range of 10-30 μm. To suspend these materials adequately requires the addition of a gellant such as bentonite for water-based fluids, or organically modified bentonite for oil based fluids. A soluble polymer viscosifier such as xanthan gum may be also added to slow the rate of the sedimentation of the weighting agent. However, a penalty is paid in that as more gellant is added to increase the suspension stability, the fluid viscosity (plastic viscosity) increases undesirably resulting in reduced pumpability. This is obviously also the case if a viscosifier is used to maintain a desirably level of solids suspension.
The sedimentation (or “sag”) of particulate weighting agents becomes more critical in wellbores drilled at high angles from the vertical, in that sag of, for example, one inch (2.54 cm) can result in a continuous column of reduced density fluid along the upper portion of the wellbore wall. Such high angle wells are frequently drilled over large distances in order to access, for example, remote portions of an oil reservoir. In such instances it is important to minimize a drilling fluid's plastic viscosity in order to reduce the pressure losses over the borehole length. At the same time a high density also should be maintained to prevent a blow out. Further, as noted above with particulate weighting materials the issues of sag become increasingly important to avoid differential sticking or the settling out of the particulate weighting agents on the low side of the wellbore.
Being able to formulate a drilling fluid having a high density and a low plastic viscosity is no less important in deep high pressure wells where high-density wellbore fluids are required. High viscosities can result in an increase in pressure at the bottom of the hole under pumping conditions. This increase in “Equivalent Circulating Density” can result in opening fractures in the formation, and serious losses of the wellbore fluid into the fractured formation. Again, however, the stability of the suspension is important in order to maintain the hydrostatic head to avoid a blow out. The objectives of high-density fluids with low viscosity plus minimal sag of weighting material can be difficult to reconcile. The need therefore exists for materials to increase fluid density that simultaneously provide improved suspension stability and less viscosity increase.